The present invention relates to a titanium-based composite material, which can be utilized for high-stress component members of a variety of machines, and a process for producing the same. In particular, it relates to a titanium-based composite material, which is suitable for engine valves for automobiles, etc., which are required to exhibit heat resistance, and a process for producing the same.
Since titanium alloys exhibit high specific strength and good toughness, they are used in various machinery component members. For example, with the U.S.A. and the U.K. as the central figure, titanium alloys have been used mainly in the fields of military, space and aircraft. Further, in these fields, heat resistant titanium alloys exhibiting good heat resistance have been developed energetically. However, since these heat resistant titanium alloys have been developed while being emphasized on their performances, they are expensive and lack mass-producing capability. Furthermore, it is difficult to melt and form them, and their yield rates were poor. Accordingly, these titanium materials were used only in limited fields.
However, recently, as the high-performance and light-weighting requirements of machinery are increased, titanium materials, especially titanium materials which are good in terms of heat resistance, have been given attention again in general machinery fields, such as automobile, etc. As one of the examples of the titanium materials, which are good in terms of the heat resistance, an automotive engine valve is hereinafter described.
Conventionally, engine valves are disposed in inlet ports and outlet ports of an engine, and they are an important component part which determines the performance of the engine, such as the fuel consumption, the efficiency, the output, and so on. Further, the engine valves become high temperatures exceeding 600xc2x0 C. In particular, the valves (exhaust valves) in the exhaust system become considerably higher temperatures than the valves (intake valves) in the intake system. For instance, even in a mass-produced engine, since the exhaust valves are subjected to a higher temperature, there may be a case where the exhaust valves become at around 800xc2x0 C. Therefore, the exhaust valves are required to exhibit good heat resistance. The conventional exhaust valves for mass production have used a heat resistant steel, such as SUH35, etc., as per JIS standard.
However, when the heat resistant steel, such as SUH35, is used in a reciprocating component part, like the valve, its inertial weight increases, because the specific gravity is large. Consequently, the maximum number of the revolutions is limited, further, since it is necessary to increase the spring load, the friction enlarges, and the engine is inhibited from being high performance.
Hence, it is considered to apply a titanium material, which is good in terms of the specific strength, etc., to the engine valve. Since the titanium material is light weight, and since it is superb in terms of the mechanical properties, it is a very attractive material. When the titanium material is applied to the engine valve, it is possible to reduce the inertial weight, to make it produce a higher output, and to improve the fuel consumption. Accordingly, titanium materials have been employed earlier for engine valves for racing cars.
However, in view of the costs, the titanium materials have not been employed for mass-produced engine valves. In particular, since the conventional titanium material has a working limit temperature of about 600xc2x0 C., it is difficult to employ it to the component members, like exhaust valves, which are used in elevated temperature ranges.
Next, the heat resistance of titanium materials will be investigated. Generally, the heat resistance of titanium alloys is governed by the structure. The structure is determined by the alloy composition, the processing temperature, the processing degree and the heat treatment conditions after processing. In particular, the structure is affected greatly by the processing temperature.
For example, there is a case where the heat resistance of titanium materials is enhanced by containing silicon in the titanium materials. In this case, by taking the relationship between the xcex2 transformation temperature and the solid solution temperature of a silicon compound (silicide) into consideration, it is necessary to determine the processing temperature. Specifically, in the case that the xcex2 transformation temperature is higher than the solid solution temperature of the silicide, when a titanium alloy (for example, Tixe2x80x94Alxe2x80x94Snxe2x80x94Zrxe2x80x94Nbxe2x80x94Moxe2x80x94Si-based titanium alloy) is processed by hot working at a high temperature of the xcex2 transformation temperature or more, coarse acicular microstructure has been formed. This acicular microstructure is unpreferable, because it becomes the causes of the casting breakage, the deterioration of elongation and the degradation of low cycle fatigue property.
While, the processing at the xcex2 transformation temperature or less is generally difficult, because the deformation resistance is large. It is understood from this example that the processing ability decreases when it is intended to improve the heat resistance of titanium material. Accordingly, it is difficult to obtain the compatibility between the heat resistance and the processing ability.
In order to solve such assignments, and to further improve the heat resistance, etc., of titanium materials, various proposals have been made, for instance, as follows.
{circle around (1)} Japanese Examined Patent Publication (KOKOKU) No. 4-56,097 (registered No. 1,772,182), an Alxe2x80x94Snxe2x80x94Zrxe2x80x94Nbxe2x80x94Moxe2x80x94Si-contained alloy, in which a trace amount of C is contained, is disclosed. This titanium alloy is enhanced in terms of the heat resistance, the heat treating property and the hot working property by adding a trace amount of C so that the xcex1+xcex2 region, which is the temperature range of the heat treatment and the hot working, is enlarged.
However, in the case of this titanium alloy, the temperature (working limit temperature), at which a sufficiently high temperature tensile strength and fatigue property are obtained, is 600xc2x0 C. approximately. Further, this titanium alloy is produced by melting, casting and forging, which are regarded as basic processes. Hence, the costs go up, and accordingly it is not suitable for mass-produced articles, such as automotive component parts, which are required to be low costs.
Furthermore, although the xcex1+xcex2 region is enlarged, the solid solution temperature of the silicide is lower than the xcex2 transformation temperature. Consequently, when hot working is carried out at a temperature higher than the xcex2 transformation temperature, coarse acicular microstructures have been formed. In order to avoid this, in the publication, eventually, the processing is carried out at a temperature of the xcex2 transformation temperature or less. Therefore, although the titanium alloy forms the balanced bi-modal structure in view of the material properties, it still exhibits large processing resistance, and the hot working property is not fully improved.
{circle around (2)} In Japanese Unexamined Patent Publication (KOKAI) No. 4-202,729, there is disclosed an Alxe2x80x94Snxe2x80x94Zrxe2x80x94Nbxe2x80x94Moxe2x80x94Si-contained alloy, in which Mo is added in an especially large amount, is disclosed. Thus, the heat resistance of the alloy is improved to about 610xc2x0 C.
However, even in this case, similarly to the titanium alloy of Japanese Examined Patent Publication (KOKOKU) No. 4-56,097, the heat resistance is insufficient. In addition, the addition of Mo in a large amount is unpreferable, because it causes the deterioration of the high temperature tensile strength.
Further, a titanium alloy is disclosed which further contains at least one member selected from the group consisting of C, Y, B, rare-earth elements and S in a total amount of 1%. Thus, the heat resistance, specifically, the creep resistance is improved.
However, even in this case, a sufficient creep property can be obtained up to about 600xc2x0 C. only, where the dislocation creep governs, and the heat resistance is insufficient. Especially, a sufficient creep resistance cannot be obtained in an elevated temperature range of 800xc2x0 C. approximately in which the diffusion starts contributing.
Moreover, in both of the cases, melting, casting and forging used as basic processes, lead to high costs, so that they are not suitable for mass-produced component parts, and so on.
{circle around (3)} There is a report on a titanium-based composite material in which titanium boride whiskers are composited by using the Ingot Metallurgy Process (IM) and the Rapid Solidification Process (RS) (Preparing Damege-Tolerant Titanium-Matrix Composites, JOM, November 1994, P68).
According to this literature, it is reported that good properties in terms of the strength, rigidity and heat resistance can be obtained by this titanium-based composite material.
However, the dispersion of the titanium boride whiskers are in homogenous, and the high-cycle fatigue property at elevated temperatures is low. The high-cycle fatigue property in the high temperature range, in addition to the high temperature creep property, is an important property, required for exhaust valve materials, and the like, for an automotive engine. Accordingly, it is not a material, which is suitable for exhaust valves, etc.
Moreover, the Ingot Metallurgy Process or the Rapid Solidification Process as the basic process is used for the titanium-based composite material, the costs of this titanium-based composite material go up.
Therefore, in view of the heat resistance and the costs, it is difficult to apply this titanium-based composite material as well to mass-produced component parts, such as automotive component parts, and so on.
{circle around (4)} In Japanese Unexamined Patent Publication (KOKAI) No. 5-5,142, a titanium-based composite material is disclosed which is made of a matrix, being composed of xcex1-type, xcex1-type+xcex2-type and xcex2-type titanium alloys, and 5-50% by volume of a titanium boride solid solution. The titanium boride solid solution, which is essentially less likely to react with the titanium alloy, is selected as reinforcing particles, thereby improving the strength, the rigidity, the fatigue property, the wear resistance and the heat resistance for this titanium-based composite material.
However, in this case as well, the properties of the titanium-based composite materials in a high temperature range over 610xc2x0 C. are not set forth at all.
{circle around (5)} In Japanese Patent Publication No. 2,523,556, there is disclosed a titanium valve, whose stem portion, fillet portion and head portion are fabricated by optimizing the hot working temperature and the heat treatment temperature.
This titanium valve obtains a desired structure by properly combining the hot working and the heat treatment. Thus, the heat resistance, etc., required for the engine valve, is satisfied.
However, the heat resistance is deficient in the high temperature range exceeding 600xc2x0 C. Moreover, since the stem portion, whose fatigue strength is considered important, is fabricated by hot working at a temperature lower than the xcex2 transformation temperature, it is difficult to carry out the hot working and it lacks the mass-productivity because of the existence of the xcex1-phase with high deformation resistance.
The present invention has been developed in view of the aforementioned circumstances. Namely, it is an object of the present invention to provide a titanium material, which is good in terms of the hot working property, the strength, the creep property, the fatigue property and the wear resistance.
In particular, it is an object of the present invention to provide a titanium material, which is good in terms of the heat resistance in a high temperature range exceeding 610xc2x0 C., and which has not been available conventionally.
More concretely, it is an object of the present invention to produce a titanium-based composite material, which is good in terms of the hot working property, the heat resistance, the mass-productivity, and so on, and to provide a process for producing the same.
The inventors of the present invention studied earnestly in order to solve this assignment, and, as a result of a variety of systematic experiments, which were carried out repeatedly, they completed the present invention. Namely, in a titanium-based composite material, which comprised a matrix, in which a titanium alloy was a major component, and titanium compound particles or rare-earth element compound particles, which were dispersed in the matrix, the inventors of the present invention optimized the composition of the matrix and the occupying amount of the titanium compound particles or the rare-earth element compound particles, and they thus came to invent a titanium-based composite material, which was good in terms of the hot working property, the heat resistance, the mass-productivity, and so on.
Namely, a titanium-based composite material according to the present invention is characterized in that it comprises: a matrix of a titanium alloy as a major component, containing 3.0-7.0% by weight of aluminum (Al), 2.0-6.0% by weight of tin (Sn), 2.0-6.0% by weight of zirconium (Zr), 0.1-0.4% by weight of silicon (Si) and 0.1-0.5% by weight of oxygen (O); and titanium compound particles dispersed in the matrix in the amount of 1-10% by volume.
Alternatively, a titanium-based composite material according to the present invention is characterized in that it comprises: a matrix of a titanium alloy as a major component, containing 3.0-7.0% by weight of aluminum (Al), 2.0-6.0% by weight of tin (Sn), 2.0-6.0% by weight of zirconium (Zr), 0.1-0.4% by weight of silicon (Si) and 0.1-0.5% by weight of oxygen (O); and rare-earth element compound particles dispersed in the matrix in the amount of 3% by volume or less.
Further, a titanium-based composite material according to the present invention is characterized in that it comprises: a matrix of a titanium alloy as a major component, containing 3.0-7.0% by weight of aluminum (Al), 2.0-6.0% by weight of tin (Sn), 2.0-6.0% by weight of zirconium (Zr), 0.1-0.4% by weight of silicon (Si) and 0.1-0.5% by weight of oxygen (O); and titanium compound particles dispersed in the matrix in the amount of 1-10% by volume; and rare-earth element particles dispersed in the amount of 3% by volume or less.
The aluminum, the tin, the zirconium, the silicon and the oxygen, which is contained in the matrix of the present titanium-based composite material, can preferably be solved into the titanium in their total amounts to make alloys.
The titanium-based composite material according to the present invention is good in terms of the hot working property. Additionally, it is good in terms of the strength, the creep strength, the fatigue property and the wear resistance not only at room temperature but also in the elevated temperature range exceeding 610xc2x0 C. It should be noted that it is good in terms of these properties in an extremely high temperature range exceeding 800xc2x0 C., for example. It is not necessarily clear why these excellent properties are obtained, but it is believed as follows.
The aluminum is an element, which elevates the xcex2 transformation temperature of the titanium alloy serving as the matrix, and which enables the xcex1 phase to exist in the matrix stably up to the high temperature range. Therefore, the aluminum is an element, which improves the high temperature strength of the titanium-based composite material. Moreover, the aluminum is an element, which further improves the high temperature strength and the creep property by solving into the xcex1 phase in the matrix.
However, when the content of the aluminum is less than 3.0%, the xcex1 phase of the titanium alloy is not fully stabilized in the high temperature region. Moreover, the solving amount of the aluminum into the xcex1 phase becomes insufficient. Accordingly, the improvements of the high temperature strength and the creep property are not expected so much. While, when the content of the aluminum is exceeds 7.0% by weight, Ti3Al precipitates so that the titanium-based composite material becomes brittle.
Note that, in order to securely improve the high temperature strength and the creep property, the content of the aluminum can further preferably be 4.0-6.5% by weight.
Although both of the tin and the zirconium are neutral elements, however, similarly to the aluminum, they enable the xcex1 phase to exist stably at elevated temperatures. In addition, they can improve the high temperature strength and the creep property by solving into the xcex1 phase.
When the content of the tin is less than 2.0% by weight, the xcex1 phase does not fully stabilize up to the high temperature region, and the solving amount of the tin into the xcex1 phase becomes insufficient so that the improvements of the high temperature strength and the creep property cannot be expected so much. Moreover, when the content of the tin exceeds 6.0% by weight, since the operation, which improves the high temperature strength and the creep property of the titanium alloy, saturates, and since the density enlarges, it is not an efficient composition. In order to securely improve the high temperature strength and the creep property, the content of the tin can further preferably be 2.5-4.5% by weight.
When the content of the zirconium is less than 2.0% by weight, the xcex1 phase does not fully stabilize up to the high temperature region, and the solving amount of the zirconium into the xcex1 phase becomes insufficient. Accordingly, the improvements of the high temperature strength and the creep property cannot be expected so much. When the content of the zirconium exceeds 6.0% by weight, since the operation, which improves the high temperature strength and the creep property of the titanium alloy, saturates, it is not an efficient composition. In order to further improve the high temperature strength and the creep property, the content of the zirconium can further preferably be 2.5-4.5% by weight.
Silicon is an element, which can improve the creep property by solving into the titanium alloy. Conventionally, the anti-creep property has been secured by solving a large amount of silicon. However, when a titanium alloy containing a large amount of silicon is held at elevated temperatures for a long period of time, the silicon combines with the titanium and the zirconium to precipitate fine silicides, and the toughness thereafter was decreased at room temperature. The present titanium-based composite material can decrease the content of the silicon, which has been required conventionally to obtain a sufficient creep property, by having the titanium compound particles and the rare-earth element compound particles, which are stable at elevated temperatures.
When the content of the silicon is less than 0.1% by weight, the creep property does not improve sufficiently, when it exceeds 0.4% by weight, the high temperature strength decreases. In order to securely improve the creep property, the content of the silicon can further preferably be 0.15-0.4% by weight.
The oxygen allows the xcex1 phase to exist stably in a high temperature range by raising the xcex2 transformation temperature of the titanium alloy. Moreover, it is an element, which can improve the high temperature strength and the creep property by solving it into the xcex1 phase. When the content of the oxygen is less than 0.1% by weight, the xcex1 phase does not stabilize sufficiently, and the solving amount of the oxygen into the xcex1 phase is insufficient, the improvements of the high temperature strength and the creep property cannot be expected so much. When the content of oxygen exceeds 0.5% by weight, the titanium-based composite material is likely to be brittle. Note that, in order to allow the xcex1 phase to stably exist and in order to securely improve the high temperature strength and the creep property, the content of the oxygen can further preferably be 0.17-0.4% by weight.
In the titanium-based composite material according to the present invention, when the aluminium, the tin, the zirconium, the silicon and the oxygen, which are included in the matrix, are solved into the titanium, it is believed that alloying brings the aforementioned good operations.
While, the titanium compound particles and the rare-earth element compound particles are less likely to react with the titanium alloy, and are thermodynamically stable particles with respect to titanium alloy. Therefore, the titanium compound particles and the rare-earth element compound particles can be present stably in the titanium alloy even in a high temperature range.
Here, the titanium compound particles include titanium boride, titanium carbide, titanium nitride, or titanium silicide, and so on, for example. More concretely, the titanium compound particles may be compounds of TiB, TiC, TiB2, Ti2C, TiN, titanium silicide, and so on. These compound particles, when they are dispersed in the titanium-based composite material, have similar properties. And, these compound particles can be used alone, or in combination, as a reinforcement member for the titanium-based composite material.
Moreover, the rare-earth element compound particles can comprise oxides or sulfides, etc., of rare-earth elements, such as yttrium (Y), cerium (Ce), lanthanum (La), erbium (Er), or neodymium (Nd), and so on. More concretely, the rare-earth element compound particles are particles, which include a compound, such as Y2O3, etc. These particles, when they are dispersed in the titanium-based composite material, have similar properties. And, these compound particles can be used alone, or in combination, as a reinforcement member for the titanium-based composite material. Note that the titanium compound particles or the rare-earth element compound particles can contain an alloying element, which constitutes the matrix.
The titanium compounds, to begin with TiB, or the oxides or sulfides,etc., of the rare-earth element are compounds, which can stably exist in the titanium alloy up to elevated temperatures. Only the compounds, which can be stably present at elevated temperatures, can inhibit the xcex2 grain growth to improve the hot working property, and can further improve the strengths at room temperature and elevated temperatures, the creep property, the fatigue property and the wear resistance.
For instance, let us take up titanium boride particles (TiB) as an example, the titanium boride particles work effectively in the improvements of the high temperature strength and the elongation. This is also disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-5,142, and so on. Accordingly, when the titanium boride particles are dispersed in the matrix, it is possible to improve the strength, the creep property, the fatigue property and the wear resistance of the titanium-based composite material, not only in the ordinary temperature range, but also in the high temperature range.
Here, the hot working property of the titanium-based composite material according to the present invention is remarked additionally. Usually, when a titanium alloy is heated to the complete xcex2 region and the hot working is carried out, xcex2 grain is coarsened, and cracks, etc., are likely to take place in the hot working, the limit upsetting ratio (a minimum upsetting ratio at which cracks take place by carrying out the upsetting.) decreases. With respect to this, the present titanium-based composite material has the following good characteristics.
Since the titanium compound particles or the rare-earth element compound particles are dispersed finely and uniformly in the entirety of the matrix, in the case where the hot working is carried out, the titanium compound particles and the rare-earth element compound particles effectively inhibit the xcex2 grain growth. Consequently, the titanium-based composite material according to the present invention comes to have a good hot working property, because no cracks take place even when the hot working is carried out at a temperature of the xcex2 transformation or more.
Especially, in the case where the titanium-based composite material according to the present invention is obtained by the sintering method, it is convenient, because the titanium compound particles or the rare-earth element compound particles are finely and uniformly dispersed in the matrix. And, since the titanium compound particles and the rare-earth element compound particles are hardly precipitated in the interface, the present titanium-based composite material comes to have a much better hot working property.
Of course, the production process for the titanium-based composite material according to the present invention is not limited to this. For example, there are the melting casting process, the rapid solidification process, etc. However, when the sintering process is used, it is good in all aspects, such as, the costs, the productivity, the material property, and so on.
Thus, the titanium-based composite material is preferred that the titanium compound particles and/or the rare-earth element compound particles are dispersed uniformly. Accordingly, in the case where the titanium compound particles are dispersed in the matrix, it is necessary for the titanium compound particles to occupy 1-10% by volume when the entire volume of the titanium-based composite material is taken as 100% by volume.
When the occupying content of the titanium compound particles is less than 1% by volume, the occupying content is too small, so that the titanium-based composite oxide cannot acquire the sufficient high temperature strength, the creep property, the fatigue property and the wear resistance. While, when it exceeds 10% by volume, the toughness has deteriorated.
Alternatively, in the case where the rare-earth element compound particles are dispersed in the matrix, it is necessary for the rare-earth element compound particles to occupy 3% by volume or less when the entire volume of the titanium-based composite material is taken as 100% by volume. When it exceeds 3% by volume, the toughness has deteriorated.
Hence, in the titanium-based composite material according to the present invention, the volume occupying contents of the titanium compound particles and rare-earth element compound particles are, respectively, 1 to 10% by volume and 3% by volume or less with respect to the entirety. With this arrangement, the present titanium-based composite material can fully improve the high temperature strength, the rigidity, the fatigue property, the wear resistance and the heat resistance without degrading the toughness.
Further, in order to further improve these properties, it is further preferred that the titanium compound particles are 3-7% by volume, or that the rare-earth element compound particles are 0.5-2% by volume.
As having described so far, in the titanium-based composite material according to the present invention, along with the hot working property, the superb properties can be obtained in terms of the strength, the creep property, the high-cycle fatigue property and the wear resistance. In particular, these properties are also good in a high temperature region, which exceeds 610xc2x0 C.